Ship to Ship Transfer Operations (sometimes called "Lightering", Lightening", or simply "S.T.S.") take place frequently, on many parts of the world's oceans, between very different types of ships, and in very varied environmental conditions. Different techniques are used to suit the prevailing conditions. These notes will look at some of the Shiphandling problems involved in such operations.
S.T.S. Operations on an Anchored Ship - in this case, one ship (normally the larger) remains at anchor throughout the operation, allowing the second ship (normally smaller, and more maneuverable) to berth alongside. The main ship handling problem during this berthing operation will probably be Yawing of the Anchored Ship. Such Yawing can make safe berthing very difficult, particularly if a combination of some of the following unfavorable factors is present:-
a) The Anchorage is an exposed open roadstead.
b) Strong and Conflicting Environmental Conditions of Wind, Sea, and Current.
c) The use of Large Ships.
d) The Manoeuvring Characteristics of the Berthing Vessel being unsuitable for the task. (Suitable Manoeuvring Characteristics for such a Vessel could include - size below 100,000 Dwt, Bridge Control of Main Engine, Controllable Pitch Propeller, High Engine Power in relation to Dwt, Adequate Steerageway at Low Minimum Speed, and a Powerful Bow Thruster).
If a wrong combination of the above factors is present, then a difficult operation could become potentially very dangerous; it would be advisable to consider the alternatives, of either postponing the operation or carrying out the berthing with both ships underway. Some aspects of berthing, with both ships underway, are described below.
Control of S.T.S. Operations - it is recommended that an officer, experienced in S.T.S.Operations, is appointed to control the conduct of the operation. This officer could be the Master of one of the two ships; alternatively, he could be a "Lightering Master", specially allocated to the Berthing Vessel for the operation. He normally needs to make the following action before the "Berthing phase" can start:-
a) Arrange a rendezvous for the operation, where there is sufficient sea room,
b) Arrange and check efficient communications between ships.
c) Ensure that suitable Yokohama fenders are provided and rigged upon one of the ships.
d) Ordering the most suitable "Lightering Course" for the operation; this will depend on prevailing weather conditions and the characteristics and state of loading of the two ships.
e) Determining the "Lightering Speed" for the operation; this will normally be the minimum speed at which both ships are able to maintain steady engine revs and steerageway. During the "Berthing phase", the "Guide"(normally the larger ship) will need to steer the "Lightering Course" at steady revs, to maintain the "lightering speed" - these revs. should not be altered except in an emergency.
f) Ensuring that S.T.S. procedures are thoroughly understood aboard both ships by Master. Officers, and Crew.
Dangerous Positions during S.T.S.Operations - it is important to be aware of two potentially dangerous positions, which need to be avoided during the "berthing phase".
a) Diagram "X" shows the starboard bow of the Berthing Vessel getting too close to the starboard quarter of the Guide. Strong Interaction forces will tend to attract the bow of the Berthing Vessel into the side of the Guide - at the same time, the Guide will tend to be turned to Starboard across the bows of the Berthing Vessel.
b) Diagram "Y" shows that the Berthing Vessel has overshot the Guide during the final approach. Strong Interaction forces will now tend to attract the bows of the Guide into the after a section of the hull of the Berthing Vessel - at the same time, the Berthing Vessel will tend to be turned across the bows of the Guide.
To avoid these two dangerous positions two distinct methods of approach, and berthing have been developed, namely:-
i) The Parallel Approach - a safer slower method.
ii) The Angled Approach - a faster method, where considerable experience and expertise are required.
The Parallel Approach - see diagram A. The Berthing Vessel approaches the Guide from the quarter, and "takes station" about a ship's length off- this avoids passing through the dangerous Interaction area around the Guide's stern. The Berthing Vessel does not attempt to close the Guide until the following conditions are matched to the Guide:-
The Berthing Vessel now starts to close the Guide at a fine angle of some 10 degrees or less. As the distance closes, care must be taken for both Speed and Position of the Berthing Vessel to match that of the Guide. Interaction Forces around the Bows of each vessel will tend to oppose contact between the ships. It is essential for the Guide to use the effective rudder to counteract this force and maintain a steady course. The Berthing Vessel will need to use the rudder to drive the ships together.
The Berthing Vessel should aim to make "first contact" on the forward shoulder fender, and still maintaining the approach course at a fine angle to that of the Guide. Immediately after "the first contact" there will be a tendency for the Berthing Vessel to bounce off the shoulder fender; this tendency will be reinforced by Interaction pressure forcing the Bows apart, and Interaction suction bringing the sterns together. At this stage, the Berthing Vessel will need to be prepared to use the considerable port rudder to stop the bows separating, and a gap opening between the two ships.
It is considered advisable for the Berthing Vessel to remain bows into the Guide at a fine angle until all the forward mooring lines and springs are secured. The Berthing Vessel can then ease gently parallel to the Guide assisted by Interaction suction at the stern. After mooring lines and springs can now be secured.
Once the Berthing Vessel is fully secured to the Guide it is normal practice to proceed to anchor the larger ship, prior to cargo transfer. However, circumstances may favor transferring cargo underway.
The Angled Approach - see Diagram B. This method is faster than the Parallel Approach but does require considerable expertise and experience, and thus is potentially more dangerous.
The Berthing Vessel closes the Guide from the quarter at a fine angle, aiming to make "first contact" on the forward shoulder fender; the Berthing Vessel thus needs a slightly greater speed than the Guide during this approach. The Berthing Vessel needs to maintain this speed advantage when passing through the Guide's quarter wake, and then through the dangerous Interaction area around the Guide's stern.
It is then necessary to reduce the Berthing Vessel's speed, so that by the time of "the first contact", the two ships speeds are matched. This is where expertise is required, as effective steerageway must be maintained.
Once "first contact" is made, then the procedures described under the Parallel Approach apply.
Precautions during Cargo Transfer - Careful attention needs to be paid to Mooring Lines during the transfer of Cargo, due to potential changes in the following factors:-
a) Weather Conditions.
b) Change of Tide.
c) The freeboards of both Vessels will be changing.
d) Stability changes may well affect the Rolling Periods of both vessels.
Unberthing - on completion of Cargo transfer it is important that the Unberthing Operation is carefully planned, and safely executed. In particular, the following points need consideration: -
a) The Freeboards of both Vessels will have changed considerably since the Berthing.
b) The Sequence for letting go, Mooring Lines, needs careful consideration, particularly when only small deck crews are available.
c) All Personnel involved need thorough Briefing.
d) The Departing Vessel will probably be fully laden. Powerful Engine movements will be required to get her moving initially, particularly when Wind and/or Current is present.
e) Departure can be made, either with both vessels underway or from one ship at anchor; the actual method will depend on local conditions and procedures.
f) When all Mooring Lines have been let go, Interaction Forces may cause both Vessels to remain locked together. In such circumstances care, patience, and skilled planned ship handling are needed to ensure a safe departure.
g) The departure Procedure may involve "lifting off" the bow of the departing vessel. Consideration needs to be given to the effect of the vessels' sterns closing - Boats and Bridge Wings can come into contact, particularly when ships are Rolling in a seaway. Appropriate fendering needs to be in place.
DANGEROUS POSITIONS IN LIGHTERING